1. Field of the Invention
The present invention relates to a crystallization apparatus and a crystallization method. Specifically, the present invention relates to an apparatus and a method for forming a crystallized semiconductor film by irradiating a polycrystalline semiconductor film or an amorphous semiconductor film with a laser beam whose phase is modulated using a phase-shift mask.
2. Description of the Related Art
Conventionally, the materials of a thin-film transistor (TFT) used for, e.g., a switching element that controls a voltage to be applied to pixels of a liquid crystal display (LCD) are roughly divided into amorphous silicon and polysilicon. The electron mobility of the polysilicon is higher than that of the amorphous silicon. If, therefore, a transistor is formed by polysilicon, its switching speed becomes higher than that of a transistor formed by amorphous silicon, which brings about the advantages that the response speed of a display can increase and the design margins of other components can decrease. If, moreover, peripheral circuits such as a driver circuit and a DAC as well as a display main unit are incorporated into the display, they can be operated at higher speed.
Though polysilicon is formed of a set of crystal gains, its electron mobility is lower than that of image crystal silicon. A small-sized transistor that is formed by polysilicon has a problem of variations in the number of crystal grain boundaries in a channel section. A crystallization method for generating large-diameter image crystal silicon has recently been proposed in order to improve the electron mobility and lessen the variations in the number of crystal grain boundaries in a channel section.
As a crystallization method of this type, there has conventionally been known phase control excimer laser annealing (ELA) for forming a crystallized semiconductor film by applying an excimer laser beam to a phase-shift mask that is parallel and close to a polycrystalline semiconductor film or an amorphous semiconductor film. The details of the phase control ELA are disclosed in, for example, Surface Science, Vol. 21, No. 5, pp. 278-287, 2000. The same technology is also disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-306859 (KOKAI date: November 2).
The phase control ELA generates a light intensity distribution of an inverted peak pattern in which the light intensity of a portion corresponding to a phase-shift section of the phase-shift mask is approximately zero (a pattern of light intensity which is approximately zero in the center of the phase-shift mask and suddenly increases toward the periphery thereof). A polycrystalline semiconductor film or an amorphous semiconductor film is irradiated with light having the light intensity distribution of an inverted peak pattern. As a result, a melting region is generated in accordance with the light intensity distribution, and a crystal nucleus is formed in a portion unmelted or solidified first in accordance with the portion whose light intensity is approximately zero. Crystal grows laterally from the crystal nucleus toward its periphery (lateral growth) to generate a large-diameter single crystal.
As described above, prior art (proxy method) for forming a crystallized semiconductor film by applying an excimer laser beam to a phase-shift mask that is parallel and close to a semiconductor film is known. However, the proxy method has a drawback in which the phase-shift mask is contaminated due to abrasion of the semiconductor film thereby to prevent good crystallization.
Applicant of the present application therefore proposes a technology (image defocus method) of forming a crystallized semiconductor film by defocusing a semiconductor film with respect to an image-forming optical system that is interposed between a phase-shift mask and a substrate to be processed (semiconductor film). Applicant also proposes a technology (image NA method) of arranging an image-forming optical system to optically conjugate a phase-shift mask and a semiconductor film and forming a crystallized semiconductor film by defining the numerical aperture of the image-forming optical system on its laser beam emitting side.
According to the above image defocus method and image NA method, generally, a KrF excimer laser light source that outputs a KrF excimer laser beam of a high output is employed and so is an image-forming optical system that is made of a single type of optical material (e.g., silica glass). In this case, the oscillation wavelength band of the KrF excimer laser beam is relatively broad (about 0.3 nm at full width at half maximum: see FIG. 5), and the image-forming optical system causes a relatively large chromatic aberration. Consequently, an influence of the broadband wavelength of the KrF excimer laser beam and the chromatic aberration of the image-forming optical system can prevent a required light intensity distribution from being generated on a substrate to be processed, such as a semiconductor film. In order to generate a required light intensity distribution on a semiconductor film, it can be thought that the waveband of a laser beam is narrowed using a prism, a diffraction grating or the like. If, however, the waveband of a laser beam is narrowed, a laser unit will be increased in size and complicated and its output will be lowered.
Using a laser beam whose waveband is narrow, generally, the light intensity distribution in a middle portion M between adjacent two inverted peak pattern portions R is accompanied by an irregular swell (see FIG. 4). In the crystallization process, a crystal nucleus is sometimes formed in a position of the swell of the middle portion M where the light intensity is low (or in an undesired position). Even though a crystal nucleus is formed in a desired position, the lateral growth, which starts from the crystal nucleus toward the periphery thereof, stops in a position of the middle portion M where the light intensity decreases. It is thus likely that a large crystal will be prevented from growing.
An object of the present invention is to provide a crystallization apparatus and a crystallization method capable of forming a crystal nucleus in a desired position and allowing a crystal to grow in a fully lateral direction from the crystal nucleus to thereby generate a large-diameter crystal grain.